refactor(js): unify env lifecycle refcounting

[nazarhussain]

Motivation

export_module.zig (bare atomic) and js/io.zig (mutex-guarded refcount) duplicate the same retain/release lifecycle. That split is the root cause of both open fixes: the cross-env init race (#31) and the missed cleanup on registration failure (#52).

Description

• Add lifecycle.SharedResource(T, hooks): a mutex-guarded refcount that runs fallible init under the lock before incrementing, and cleanup after decrementing.
• Port js/io.zig and the module lifecycle in export_module.zig onto it. No public API changes.

Both fixes hold by construction:

• Hooks are serialized per module, and the refcount only increments after init succeeds — fixes the fix(js): serialize moduleInit/cleanupHook across N-API envs #31 race, with registration kept outside the lock.
• Rollback goes through release(), which always runs cleanup with the correct count — fixes fix(js): roll back module lifecycle initialization #52.

Supersedes #31 and #52.

Testing

New tests in lifecycle.zig cover hook ordering, init-failure rollback, and instance visibility. zig build test: 54/54 pass.

🤖 Generated with Claude Code

[nazarhussain]

@GrapeBaBa The changes in #31 and #52 were using the pattern and logic we already have in io.zig so I extracted it out to a common file lifecycle.zig and used across both io.zig and export_module.zig.

See if that seems good to you.